(a) Field of the Invention
The present invention relates to a telephone station which allows switch-over between dialling signal transmissions in pulse code and dialling signal transmissions in multifrequency code. This telephone station may be used as a data transmitter as well as a conventional telephone set which is connected to a telephone subscriber's line.
A such telephone station comprises a pulse code signal generator which produces decimal code pulse trains associated respectively with the numerals of the keys of the station keyboard and a multifrequency code signal generator which produces multifrequency code a.c. signals associated respectively with the keyboard keys.
(b) Description of the Prior Art
It is known that such a telephone station is designed to give domiciliary access to data processing and in addition to normal telephone communications with other telephone subscriber's stations also sets up telephone communications with data terminals. Generally, there are two method types for communicating with a data terminal, depending upon whether the linking telephone exchange possesses multifrequency code receivers or not. The first method consists of transmitting both the terminal call dialling phase and the data transmission phase in multifrequency code. The second method consists of transmitting the call dialling phase in decimal pulse code, then the data transmission phase in multifrequency code. The pulse code signals consist of decimal code pulse trains which are associated respectively with the keys of the keyboard and are generated by opening the loop of the telephone line which connects the telephone station to the exchange. Each multifrequency code signal represents the combination of two voice-frequencies out-of-8 which are associated with the keys of the keyboard respectively and are transmitted over the telephone line.
Known telephone stations accomplish the first method or the second method in setting up a telephone communication by manually disconnecting the pulse generator from the telephone line when the telephone station is connected to a telephone exchange which accepts only dialling signals completely in multifrequency code. For example, the telephone station described in U.S. Pat. No. 3,920,926 to George V. Lenaerts et al comprises in a first embodiment only a multifrequency generator for setting up communications in accordance with the first method, whereas in a second embodiment it comprises a multifrequency code a.c. signal generator and a decimal code pulse generator for setting up only communications in accordance with the second method. More advantageously, the telephone station disclosed in U.S. Pat. No. 3,988,549 to Jean-Claude Merlin et al comprises a multifrequency code a.c. signal generator and a decimal code pulse generator which is disconnected from the telephone line by disconnecting a connection strap for example when communications have to be set up in accordance with the first method.
In the case of telephone stations in which the second method is carried out, switching over from the pulse code to the multifrequency code is done either manually by means of an appropriate key on the keyboard which when pressed directly causes disconnection of the pulse generator from the telephone line after the complete call digit dialling, as described for example in the U.S. Pat. No. 3,920,926 or automatically on reception of a predetermined signal transmitted from the linking exchange to the telephone station after the call digit dialling. When the subscriber hangs up, the pulse generator is again connected to the telephone line. The predetermined signal is issued for example on detecting the reversal of the telephone station supply current, which happens in the majority of telephone exchanges when a communication has been set up after decimal dialling, as described in the U.S. Pat. No. 3,988,549, or on detecting a signal having a predetermined frequency which differs from the voice-frequencies defined by the multifrequency code and which is transmitted from the linking exchange over the telephone line between the telephone subscriber's station and the data terminal, as described in German Patent Application No. P 2,240,979 filed on Aug. 21, 1972.
Consequently, according to the prior art, each type of telephone exchange has to have connected to it a telephone station of a corresponding predetermined type which comprises for example a battery reversal detector or a predetermined frequency signal detector. This does not allow such a telephone station to be connected to the telephone exchanges of certain countries which do not carry out battery reversal or which do not include a predetermined frequency signal generator. This limits considerably the application of such telephone stations. Furthermore, automatic switching over of the multifrequency code and decimal pulse code must be authorized after a predetermined phase in setting up a communication and this does not allow a communication to be set up between such a telephone station and a data terminal by way of any present day public telephone network.
In this respect, in certain cases, according to the transmission features of the chain of telephone exchanges which make the telephone connection, one actually provides for the transfer from the pulse code to the multifrequency code that it can be made during the call digit dialling, for example after transmitting one of the international and national prefixes or linking or main telephone exchange code prefix. In this case, the aforesaid telephone stations do not authorize automatic switching of the codes at a determined phase in each communication, because for example the battery reversal or the detection of a predetermined frequency must be accomplished after the completion of the call dialling. Consequently, when the automatic switching of the codes must be accomplished, the telephone stations of the prior art, in accordance with the second method, will continue to transmit in pulse code. This will be interpreted in the exchange as an opening of the loop analogous to an on-hook and such communications will not be able to be set up.